A display unit using organic EL, and in particular, a display unit wherein organic EL layers are driven by circuits constituted by thin film transistors (TFT), is commonly known in the prior art. For example, the paper presented by Shimoda, et. al., (T. Shimoda, H. Ohshima, S. Miyashita, M. Kimura, T. Ozawa, I. Yudasaka, S. Kanbe, H. Kobayashi, R. H. Friend, J. H. Burroughes and C. R. Towns: Proc. 18th Int. Display Research Conf., Asia Display 98, (1998) p. 217), discloses a method wherein a driving circuit using a low-temperature polysilicon (poly-Si) thin-film transistor (TFT) is formed on a glass substrate for each picture element, whereupon a wiring forming step, a transparent electrode forming step, a bank layer forming step, a positive hole injection layer forming step, an organic EL layer forming step, a cathode forming step, and the like, are performed, thereby creating an organic EL display unit.
FIG. 16 and FIG. 17 show the composition of a display unit created according to commonly known technology of this kind. FIG. 17 is a plan view of the organic EL display unit created according to commonly known technology, and FIG. 16 is a sectional view along section B-B (curved section) in the plan diagram shown in FIG. 17. As shown in FIG. 16, a thin film transistor 2, wiring layer 3, transparent electrode 4, bank layer 5, positive hole injection layer 6, organic EL layer 7, and cathode 8 are deposited successively on a glass substrate 1.
Here, since the cathode 8 is formed by a metal which does not transmit light, the light from the organic EL layer 7 is extracted externally from the side of the glass substrate 1 on which the driving circuits are formed. In other words, the face on the side of the driving circuits forms a front surface with respect to the organic EL layer 7. In a display unit of this kind, since the region where the driving circuits are formed does not transmit light, the aperture ratio declines. In other words, as shown in FIG. 17, it is necessary to form an organic EL layer 7 which avoids the region where the thin film transistor 2 and other wires (capacitor 2, wires 3 and 9) are formed. If it is required to increase the performance or added value of the display unit by incorporating various circuitry, such as memory circuits, or the like, into the picture element regions, or if it is attempted to achieve a finely detailed display unit, then since the surface area of the circuit region which does not transmit light becomes relatively large, the aforementioned decline in the aperture ratio becomes a marked problem.
In order to resolve this problem, it is necessary to use a structure wherein driving circuits, or the like, are not present on the side from which the light is emitted, in other words, a structure which uses a transparent electrode material for the cathode or which places the cathode on the driving circuit side.
However, there is a problem involved in using a transparent material for the cathode. This is due to the limitation whereby a material which is close to the work function of the organic EL material used in the organic EL layer must be selected for the electrodes. For example, for the electrode material used in the anode, it is necessary to select a material which is close to the work function of the HOMO level of the organic EL material, and for the electrode material used in the cathode, it is necessary to select a material which is close to the work function of the LUMO level of the organic EL material. However, at the present time, there is no suitable transparent electrode material which is close to the LUMO level of the organic EL material. It has been proposed that the cathode film be formed extremely thinly, but thin electrode layers give rise to faults in terms of durability and current capacity, and are not desirable from the viewpoint of reliability.
On the other hand, in the case of a structure where the cathodes are provided on the driving circuit side, until now, it has been necessary to form an organic EL layer after forming the cathodes, and then to form a positive hole injection layer thereon. In this case, since it is necessary to from the organic EL layer before the positive hole injection layer, there is a risk of lack of uniformity in the thickness of the organic EL layer, and hence unevenness in the amount of light emitted may occur. Moreover, since the material used for the cathode is a material that is liable to oxidization, such as calcium, or the like, then the cathode must be formed with a sealed structure. In view of these circumstances, it is difficult to extract light from the organic EL layer on the opposite side to the driving circuits.